This application claims priority from Finland Application No. 19992356, entitled xe2x80x9cPlanar Antenna,xe2x80x9d filed on Nov. 1, 2000, the disclosure of which is hereby incorporated by reference in its entirety.
1. Field of the Invention
The invention relates in particular to a planar antenna structure installable inside small-sized radio apparatus.
2. Description of the Related Art
In portable radio apparatus it is very desirable that the antenna be placed inside the covers of the apparatus, for a protruding antenna is impractical. In modem mobile stations, for example, the internal antenna naturally has to be small in size. This requirement is further emphasized as mobile stations become smaller and smaller. Furthermore, in dual-band antennas the higher operating band at least should be relatively wide, especially if the apparatus in question is meant to function in more than one system utilizing the 1.7-2 GHz band.
When aiming at a small-sized antenna the most common solution is to use a PIFA (planar inverted F antenna). The performance of such an antenna functioning in a given frequency band or bands depends on its size: The bigger the size, the better the characteristics, and vice versa. For example, decreasing the height of a PIFA, i.e. bringing the radiating plane and ground plane closer to each other, markedly decreases the bandwidth and degrades the efficiency. Likewise, reducing the antenna in the directions of width and length by making the physical lengths of the elements smaller than their electrical lengths decreases the bandwidth and especially degrades the efficiency.
FIG. 1 shows an example of a prior-art dual-band PIFA. In the Figure there can be seen the frame 110 of the apparatus in question which is drawn horizontal and which functions as the ground plane of the antenna. Above the ground plane there is a planar radiating element 120 which is supported by insulating pieces, such as 105. Between the radiating element and ground plane there is a short-circuit piece 102. The radiating element 120 is fed at a point F through a conductor 103 via a hole in the ground plane. In the radiating element there is a slot 125 which starts from the edge of the element and extends to near the feed point F after having made two rectangular turns. The slot divides the radiating element, viewed from the feed point F, into two branches A1 and A2 which have different lengths. The longer branch A1 comprises in this example the main part of the edge regions of the radiating element, and its resonance frequency falls on the lower operating band of the antenna. The shorter branch A2 comprises the middle region of the radiating element, and its resonance frequency falls on the upper operating band of the antenna. The disadvantage of structures like the one described in FIG. 1 is that the tendency towards smaller antennas for compact mobile stations may degrade the electrical characteristics of an antenna too much; the bandwidth of the higher resonance band may be insufficient, for example.
From the prior art it is not known solutions that would significantly increase the bandwidth of a PIFA without increasing the size of the antenna. From earlier applications it is known to the applicant a structure in which the bandwidth is increased by making the slot of the radiating element in two portions having a certain ratio of widths (FI 991807), as well as a structure in which the bandwidth is increased by adding above the radiating plane a second radiating plane and by placing dielectric material between these planes and on top of the uppermost plane (FI 992268).
In the solution disclosed herein the bandwidth of a PIFA is increased by increasing in a certain area the capacitance between the ground plane and radiating plane by means of conductors. Such increasing of capacitance is known per se in the prior art. FIG. 2 shows a simplified example in which the radiating plane 220 has been bent at its edge towards the ground plane 210. Between the bend 215 and ground plane there is then a certain additional capacitance C. FIG. 3 shows a structure known from publication U.S. Pat. No. 5,764,190 where there is between the radiating plane 320 and ground plane 310 a relatively small parallel plane 315 in galvanic contact with the former to increase the capacitance. In these cases, the structural part increasing the capacitance is at the opposite end of the antenna in relation to the feed place determined by the feed conductor 203 (303) and short-circuit conductor 202 (302), and the purpose of the structural part is mainly to reduce the physical size of the antenna.
The object of the invention is to increase in a novel manner the bandwidth of a small-sized PIFA. A structure according to the invention is characterized by what is expressed in the independent claim 1. Some preferred embodiments of the invention are presented in other claims.
The basic idea of the invention is as follows: A conventional PIFA-type structure is extended by forming the structural part adding to the capacitance between the radiating plane and ground plane relatively close to the feed point of the antenna. The structural part may be a projection pointing from the radiating plane to the ground plane or vice versa.
An advantage of the invention is that it achieves a significant increase in the antenna bandwidth without increasing the size of the antenna. Another advantage of the invention is that the structure according to it is simple and the increase in the manufacturing cost is relatively low.